1. Field of the Invention
The present invention relates to a method of forming an electrode film, and more particularly, to a method for decreasing the electrical resistivity of an electrode film for use with a resonator, a filter, or other electronic component.
2. Description of the Related Art
In a radio frequency resonator (surface acoustic wave device), a band-pass filter or other filter using a Love wave or shear horizontal (xe2x80x9cSHxe2x80x9d) wave, a refractory metal is used to form an electrode film. The electrode film used for such application is required to have a low electrical resistivity, a high electromechanical coupling coefficient, and a high density. This is because the electrode film made of a refractory metal material is advantageous for suppressing deterioration in an insertion loss when increasing the frequency of a device, as compared with other materials.
It is known that when depositing such an electrode film made of a refractory metal on a piezoelectric substrate, the electrical resistivity of the electrode film rapidly increases depending upon pressure when deposition of the refractory metal is started at a back pressure of about 1xc3x9710xe2x88x924 Pa in a deposition chamber. For example, FIG. 4 shows the relationship between the back pressure (pressure in a vacuum chamber in which deposition is started) and the resistivity of an electrode film in depositing the electrode film made of a refractory metal such as tungsten. In this example, the resistivity is 13.4 xcexcxcexa9xc2x7cm at a back pressure of 1.10xc3x9710xe2x88x925 Pa, 16.4 xcexcxcexa9xc2x7cm at a back pressure of 1.40xc3x9710xe2x88x924 Pa, and 32.5 xcexcxcexa9xc2x7cm at a back pressure of 4.00xc3x9710xe2x88x924 Pa. It can be confirmed that the electric conductivity of the electrode film deposited under a pressure of 1xc3x9710xe2x88x924 Pa or more is higher than the electrical resistivity of the electrode film deposited under a pressure of about 1xc3x9710xe2x88x925 Pa or lower.
Therefore, deposition of an electrode film made of a refractory metal is conventionally started after the deposition chamber is evacuated to 3xc3x9710xe2x88x925 Pa or lower.
A load lock system vacuum deposition apparatus is suitable for depositing an electrode film using a refractory metal under a high vacuum of 3xc3x9710xe2x88x925 Pa or less, as described above. However, such a load lock system deposition apparatus has the disadvantage that the apparatus is expensive, and the number of-the substrates that can be input per batch is small, thus increasing the deposition cost.
On the other hand, a batch system vacuum deposition apparatus is inexpensive and is capable of processing a large number of substrates per batch. However, such a batch system vacuum deposition apparatus requires being exposed to air each time a substrate is exchanged for a new substrate, and the time required for increasing the degree of vacuum increases as the degree of vacuum increases, thereby causing the problem of lengthening the exhaust time for deposition under 3xc3x9710xe2x88x925 Pa or less so as to greatly decrease the throughput and manufacturing efficiency.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of forming an electrode film to produce an electrode film having low electrical resistivity even when a refractory metal is deposited under a back pressure of about 1xc3x9710xe2x88x924 Pa or more.
According to one preferred embodiment of the present invention, a method of forming an electrode film includes starting deposition of a refractory metal such as tungsten, tantalum, or other suitable material on a substrate under a back pressure of about 1xc3x9710xe2x88x924 Pa to 5xc3x9710xe2x88x923 Pa, and then annealing the substrate on which the metal is deposited to decrease the electrical resistivity of the electrode film.
It is conventionally thought that when deposition of a refractory metal is started under a pressure of about 1xc3x9710xe2x88x924 Pa or more, the electrical resistivity of the electrode film is increased to be beyond the allowable range.
As a result of intensive research of the cause for the increase in the electrical resistivity, the inventors of the present invention discovered that in the electrode film deposited under a pressure of about 1xc3x9710xe2x88x924 Pa or more, the increase in the electrical resistivity is caused by hydrogen and an oxide contained in the electrode film. It was also discovered that the hydrogen and oxide contained in the electrode film can be removed as water and hydrogen gas from the electrode film by annealing under vacuum, thereby decreasing the electrical resistivity of the electrode film.
Therefore, in accordance with preferred embodiments of the present invention, even when deposition is started under a pressure of about 1xc3x9710xe2x88x924 Pa or more, the resistivity can be decreased by annealing under vacuum, thereby decreasing an insertion loss and the deposition cost of the electrode film.
According to preferred embodiments of the present invention, after deposition is started at a back pressure of about 1xc3x9710xe2x88x924 Pa to 5xc3x9710xe2x88x923 Pa, vacuum annealing is performed to decrease resistivity, thereby decreasing the insertion loss of a resonator, a band-pass filter, or other component, and improving the efficiency of production thereof to decrease the deposition cost of an electrode film.
Other features, steps, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.
For the purpose of illustrating the present invention, there is shown in the drawings several forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.